Proper Actuator Sizing for Rotary Control Valves - Part 2 - Valve World

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Keywords: Actuator sizing • Rotary control valve • Safety

C O N T RO L & S M A RT V A LV E S

Proper actuator sizing for rotary control valves Part 2 In the sizing of rotary control valves, cost pressure leads to the actuators being sized with the lowest possible safety factors so that smaller actuator sizes can be used. The problem is that no applicable sizing standards to guarantee reliable actuator sizing exist. This results in the actuators being either oversized or undersized. This second part of the article, continues to show how rotary valves can be sized reliably. By Domagoj Vnucec, Nadine Wetzstein & Dr. Jörg Kiesbauer, Samson AG

3. Determining valve-specific coefficients Valve-specific parameters are used in the mentioned equations (2) to (5); theoretical values are not readily available for all of these parameters. Rather, these coefficients need to be determined empirically. For example, the static coefficients of friction of the packings, bearings and seat elements can be determined with relative ease using appropriate test setups. Concerning the seat friction, it is not only the maximum in the valve‘s closed position that needs to be taken into account; the course of the coefficient of friction depending on the opening angle, which is expressed in coefficient α, needs to be determined as well.

Figure 7. Comparison of measured and CFD-based flow torques.

High-performance test benches are needed to measure the dynamic coefficients (see Figure 6), for example to determine the seat friction behavior depending on the process medium and the differential pressure. In particular, the flow coefficient β, which is relevant to the dynamic flow torque, as well as the differential pressure exponent a, which depends on the geometry, need to be measured. Most often, the problem is, however, that the available test benches are not large enough for large valve sizes. Alternatively, it is possible to determine the flow coefficients using CFD. When using flow simulations, however, it is essential that calibrations with the associated measurements have been performed beforehand and that these measurements and calibrations are only applied to valves with a similar geometry. Figure 7 shows that a sufficient accuracy for the simulated flow torques can be achieved if the mesh structure has been optimized based on the mentioned calibration procedures.

4. Actuator sizing Figure 6. Water test bench of SAMSON AG at Frankfurt am Main, Germany.

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Certain criteria need to be fulfilled for reliable actuator sizing. The required torque of the valve, which is calculated

according to the presented model, serves as the basis for this. Figure 9 shows that not only the torque in the valve‘s closed position is to be considered; the valve‘s entire operating range must be taken into account when assessing an actuator. As in this example, the high torque that exists in the valve‘s open position due to the flow dynamics can assume higher values than the torque in the closing area. As a result, several safety factors need to be considered. This means that the minimum values of the spring and air torques for each opening angle need to be related to the required valve torque at the following opening angles: • •

0°: ϕPeak:

•

ϕmax:

valve’s closed position maximum torque outside closed position valve’s fully open position

To select a safety factor in the valve‘s operating range that prevents the actuator from being oversized, it is favorable if operators can provide detailed specifications of the differential pressure that exists at high valve loads. It must also be guaranteed that the maximum possible torque of the selected

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C O N T RO L & S M A RT V A LV E S

Figure 8. Sample actuator sizing for a butterfly valve and a rack-and-pinion actuator.

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C O N T RO L & S M A RT V A LV E S

Figure 9. Sample actuator sizing for a butterfly valve and a scotch yoke actuator.

actuator does not exceed the permissible torque of the valve. For the permissible torque, both the stem of the closure member as well as the connecting element between the valve and actuator are to be considered. Fluctuations in the compressed air network are to be taken into account as well when sizing actuators (see Figure 8) as fluctuating supply pressures can influence the air torques created at the actuators and thus reduce the mentioned safety factors.

5. Summary This article has shown that the frequently used method of taking only static coefficients of friction into account when calculating the required torque is not reliable. In many cases, it is the use of an indiscriminate safety factor to cover all uncertainties that leads to actuators being oversized. With valves in throttling service, it is vital to know the flow-induced torque components as not considering them results in the actuator being undersized. It is also essential to determine the associated dynamic coefficients depending

on the valve geometry and other effects either by measurement or using CFD. If the dynamic coefficients are determined using CFD, it must be taken into account that prior calibration and the associated measurements are indispensable. Measurements are also necessary to determine the static coefficients of friction. In particular, the behavior of the coefficients of friction depending on the opening angle must not be disregarded. As a result, reliable and cost-effective actuator sizing is possible when taking into account the valve’s permissible torque as well as the safety factors in the valve’s closed position and operating range.

About the authors Dipl.-Ing. Domagoj Vnucec Dipl.-Ing. Domagoj Vnucec is head of the development test bench department at Samson AG, Mess- und Regeltechnik in Frankfurt am Main, Germany. His work experience includes planning and evaluation of flow and acoustic laboratory tests carried out on control valves, application of CFD programs for the purpose of flow calculation, development and optimization of calculation and sizing methods as well as the implementation of sizing software for control valves. He can be reached at: [email protected] Domagoj Vnucec or +49 69 4009 1796.

M.Sc. Nadine Wetzstein M.Sc. Nadine Wetzstein works for the development test bench department at Samson AG, Mess-und Regeltechnik in Frankfurt am Main in Germany. Her work experience includes application of CFD programs for the purpose of flow calculation as well as planning and evaluation of flow and acoustic laboratory tests carried out on control valves. She can be reached at: [email protected] or +49 (69) 4009-2269.

Nadine Wetzstein

Dr.-Ing. Jörg Kiesbauer Dr.-Ing. Jörg Kiesbauer is the board member in charge of Research and Development at Samson AG, MESS-UND REGELTECHNIK in Frankfurt/Main, Germany. Standardization activities: Working Group 9 Final Control Elements of IEC SC 65B, DKE/K 963 Control Valves and ISA SP 75 Control Valve Standards. He can be reached at: [email protected] or +49 69 4009 1300. Jörg Kiesbauer

Bibliography (1) Kiesbauer, J., Vnucec, D.: Fields of Application for Computational Fluid Dynamics in Control Valve Development, Valve World, KCI Publishing, Volume 15, Issue 3, April 2010, p. 27 to 31 and Volume 15, Issue 4, May 2010, p. 59 to 60, 63 to 64. (2) WIB Guidline: Automated block valves (ABV) Assemblies Part 1: Valve torque requirements, May 2011. (3) Mattick, R.: Operational parameters influencing the torque of ball valves - correct actuator size selection, Valve World Conference 2006. (4) NE 14: Attachment of Pneumatic Part-Turn Actuators to Valves, NAMUR Recommendation, April 2011.

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